Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104.
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104.
Proc Natl Acad Sci U S A. 2021 Aug 3;118(31). doi: 10.1073/pnas.2100738118.
When aged below the glass transition temperature, [Formula: see text], the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid-liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of ,'-bis(3-methylphenyl)-,'-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid-liquid phase transition temperature ([Formula: see text]) ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.
当处于玻璃化转变温度以下时，[公式：见文本]，除非晶体成核，否则玻璃的密度不能超过亚稳过冷液体（SCL）状态的密度。唯一的例外是存在另一种密度高于普通 SCL 的多晶 SCL 状态。实验上，只有在形成网络的系统或具有多晶晶态的系统中，才观察到这种多晶态和相应的液-液相转变。在其他简单液体中，即使在接近 Kauzmann 温度的情况下，也没有观察到这种相转变，无论是在老化的还是气相沉积的稳定玻璃中。在这里，我们报告说，' - 双（3-甲基苯基）-，' - 二苯基联苯（TPD）的气相沉积薄膜的密度可以超过其相应 SCL 密度的 3.5%，并且在某些沉积条件下甚至可以超过晶体密度。我们确定了一种以前未被识别的高密度过冷液体（HD-SCL）相，其液-液相转变温度[公式：见文本]在普通 SCL 的名义玻璃化转变温度以下约 35 K。在厚度范围为 25 至 55 nm 的沉积玻璃中观察到 HD-SCL 状态，其中普通 SCL 的薄膜具有异常增强的表面迁移率和大的迁移率梯度。增强的迁移率使气相沉积的薄膜能够克服弛豫的动力学障碍并进入 HD-SCL 状态。HD-SCL 状态仅在薄膜中热力学有利，并且当气相沉积继续形成厚度超过 60nm 的薄膜时，它会迅速转变为普通 SCL。
